August 14, 2020, 04:56:41 PM
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Topic: Synthesis and Purification of the ester: propyl acetate (propyl ethanoate)  (Read 8583 times)

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Offline c23charles

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This seems like a simple procedure, however, after 2 attempts (the final attempt is below, the first attempt used higher quantities of base washing with an additional step of pure water wash), I have been unable to obtain any yield.  Perhaps it isn't possible to purify propyl acetate using chemical means.  Other threads have recommended distillation as a favorable method. 

Any thoughts on why this procedure didn't work?

The only think that caught my attention is that propyl acetate is 2% soluble (18.9g/L) in water.  But after 45 minutes of reflux, I should have enough ester to lose a tiny quantity in the water.

Procedure:

3 mL Glacial Acetic Acid
1 mL n-propanol
4 drops 6M Sulfuric acid

mixture refluxed at 160 C for 45 minutes

mixture transferred to sep funnel, chilled.

Added 2 mL 1M NaOH, mixed, chilled
Added 3 mL diethyl ether, mixed, chilled
Drained lower layer

Added 2 mL 1M NaOH, mixed, chilled and drained lower layer
Added 2 mL 1M NaOH, mixed, chilled and drained lower layer

Confirmed drained lower layer was pH neutral

Added 3 mL saturated salt water, mixed, chilled, and drained lower layer

Transferred mixture to large mouth beaker
Evaporated remaining diethyl ether

Stared at an empty beaker, which had no aroma except some acetic acid residue. 

Offline Honclbrif

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How are you evaporating your solvent?

Also, you should only be getting about a mL of product. With such a small quantity, it may be worth it to work it up in a test tube with a pipet and no solvent.
Individual results may vary

Offline discodermolide

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Instead of fiddling about with such small quantities scale it up by a factor of 10.
Check the BPt. of your product.
Don't use so much NaOH you may end up hydrolysing the product back to SM.
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Offline fledarmus

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How did you evaporate your ether? The boiling point of propyl acetate isn't particularly high; that is why distillation is recommended. You can ensure the propyl acetate stays where it is supposed to.

Offline c23charles

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I used a wide-mouth beaker and simply swirled it around in a fume hood while periodically dipping the bottom in a hot water bath to evaporate the ether.  It took about 3 minutes to evaporate.  I thought my nose was just "ethered out", but I thought I smelled something besides ether for a few moments before the mixture evaporated completely.

b.p. of the ester is listed at 102C, probably 93C here in Boise @ 2700 ft. 

I was a little suspicious about the 1MNaOH.  Most of the experiments I have read about use Sodium Bicarbonate to neutralize the acid.

Scaling up may be possible.  Even a factor of 10 wouldn't be a large quantity of anything, and none of these chemicals are expensive.  I can see now why having ~1mL of product could quickly evaporate with the ether during that step. 

Distillation seems problematic only because all of these things boil at approximately the same temp (I am confused about whether or not acetic acid and water are azeotropes.  I see a b.p for acetic acid at 118C, but then no azeotrope.  Shouldn't the boiling point of a non-azeotropic acid be the b.p. of the water it is dissolved in?  And then perhaps a separate b.p. for the acid?  I want to isolate the ester and come up with a yield, not just smell pear.  Perhaps I'm missing something in the possibility of distillation.

I especially like the advice of doing it in test tube with pipet (how do I keep the ester from vaporizing away?)  So I would heat the mixture for 30 minutes, and be left with a lower aqueous layer with excess acid, and an ester layer.  I take the top layer.  Done?

Any more thoughts are appreciated.  I will be doing the experiment again on Wednesday March 21, 2012. 

Thanks   

Offline discodermolide

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You scale it up as I suggested, then you use proper apparatus, not test tubes and Pasteur pipettes and open beakers for evaporation.
Then you carefully distill the solvent and then you will find product.
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Offline fledarmus

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I was a little suspicious about the 1MNaOH.  Most of the experiments I have read about use Sodium Bicarbonate to neutralize the acid.


Since the ester is in the ether layer and the sodium hydroxide is in the water layer, this shouldn't be an issue, and chilling the system makes it even less so. There is very little contact between your ester and the base - only along the interface between the two layers, and whatever additional surface area you get by mixing. Saponification isn't a very rapid reaction, and the deprotonation of acids by bases is extremely rapid. Once you have isolated your pure propyl acetate, you can demonstrate this for yourself just be redissolving it in ether, repeating the extraction, and seeing how much you recover. Essentially the only losses you will see will be from the very slight solubility of the propyl acetate in water.

Quote
Distillation seems problematic only because all of these things boil at approximately the same temp (I am confused about whether or not acetic acid and water are azeotropes.  I see a b.p for acetic acid at 118C, but then no azeotrope.  Shouldn't the boiling point of a non-azeotropic acid be the b.p. of the water it is dissolved in?  And then perhaps a separate b.p. for the acid?  I want to isolate the ester and come up with a yield, not just smell pear.  Perhaps I'm missing something in the possibility of distillation.


You aren't missing something in the distillation, but you are missing something about the purpose of your extraction. The reason that you are washing your ether with 1 M NaOH is to get rid of any acid that you have remaining after the reaction. Acid, and the n-propanol, will be washed away, leaving only the propyl acetate and ether in the organic phase. There you have ~50C difference in boiling points, which should be reasonably easy to separate by distillation.


Offline c23charles

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A procedure is starting to take shape:

1.  Add 15mL GAA + 5mL propanol + 10 drops 6M sulfuric acid.

2.  Reflux for 45 minutes, then chill mixture

3.  Add 10mL dimethyl ether

3.  neutralize aqueous layer with 1M NaOH. 

4.  chill and drain lower aqueous layer

5.  Rinse with 10mL saturated salt water?

6.  Distill off ether

Any modifications? 



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